With reference to FIG. 1, a typical turbocharger 101 has a turbine that includes a turbocharger housing and a rotor configured to rotate within the turbocharger housing around an axis of rotor rotation 103 on oil-lubricated thrust bearings and two sets of oil-lubricated journal bearings (one for each respective rotor wheel), or alternatively, other similarly supportive bearings. The turbocharger housing includes a turbine housing 105, a compressor housing 107, and a bearing housing 109 (i.e., a center housing that contains the bearings) that connects the turbine housing to the compressor housing. The rotor includes a turbine wheel 111 located substantially within the turbine housing, a compressor wheel 113 located substantially within the compressor housing, and a shaft 115 extending along the axis of rotor rotation, through the bearing housing, to connect the turbine wheel to the compressor wheel.
The turbine housing 105 and turbine wheel 111 form a turbine configured to circumferentially receive a high-pressure and high-temperature exhaust gas stream 121 from an engine, e.g., from an exhaust manifold 123 of an internal combustion engine 125. The turbine wheel (and thus the rotor) is driven in rotation around the axis of rotor rotation 103 by the high-pressure and high-temperature exhaust gas stream, which becomes a lower-pressure and lower-temperature exhaust gas stream 127 and is axially released into an exhaust system (not shown).
The compressor housing 107 and compressor wheel 113 form a compressor stage. The compressor wheel, being driven in rotation by the exhaust-gas driven turbine wheel 111, is configured to compress axially received input air (e.g., ambient air 131, or already-pressurized air from a previous-stage in a multi-stage compressor) into a pressurized air stream 133 that is ejected circumferentially from the compressor. Due to the compression process, the pressurized air stream is characterized by an increased temperature over that of the input air.
Optionally, the pressurized air stream may be channeled through a convectively cooled charge air cooler 135 configured to dissipate heat from the pressurized air stream, increasing its density. The resulting cooled and pressurized output air stream 137 is channeled into an intake manifold 139 on the internal combustion engine, or alternatively, into a subsequent-stage, in-series compressor. The operation of the system is controlled by an ECU 151 (engine control unit) that connects to the remainder of the system via communication connections 153.
U.S. Pat. No. 8,453,448, dated Jun. 4, 2013, which is incorporated herein by reference for all purposes, discloses a turbocharger similar to that of FIG. 1, but which has an axial turbine wheel. The axial turbine wheel inherently has a lower moment of inertia than an equivalent radial turbine, reducing the amount of energy required to accelerate the turbine. As can be seen in FIG. 2 of that patent, the turbine has a scroll passageway that circumferentially receives exhaust gas and (with reference to FIG. 1) axially restricts the flow to transition it to a highly swirling axial flow. It thus impacts the leading edge of the turbine blades in a generally axial and circumferential direction.
The scroll passageway of the patent is characterized by a significant enough radial reduction to accelerate exhaust gas such that a significant portion of the total pressure of the exhaust gas received by the turbine is converted into dynamic pressure. This allows an appropriately configured blade to extract a significant amount of energy from the exhaust gas without significantly changing the static pressure across the turbine blades, thus the exhaust gas stream applies little to no axial pressure on the rotor. Nevertheless, the low static pressure at the downstream end of the scroll passageway increases the potential that oil from the bearing housing could leak into the turbine housing.
Accordingly, there has existed a need for a turbocharger turbine having a low moment of inertia, and with a low likelihood of oil leaking from the bearing housing into the turbine housing. Preferred embodiments of the present invention satisfy these and other needs, and provide further related advantages.